According to a filing with the U.S. District Court in Manhattan late Tuesday, the SEC is examining whether CVR properly characterized the fees it agreed to pay financial advisers Goldman Sachs and Deutsche Bank, to help defend against Icahn's tender offer.

The SEC probe was made public in a letter from Herbert Beigel, a lawyer for CVR and Icahn.

It was filed as part of lawsuit in which CVR, now controlled by Icahn, accused law firm Wachtell, Lipton, Rosen & Katz, which had defended CVR, of malpractice for failing to disclose that Goldman and Deutsche Bank stood to earn far higher fees if a takeover defense failed than if it succeeded.

Beigel said the SEC probe began last year, and that the agency recently advised CVR it "intends to expand its inquiry."

According to court papers, the investigation would include depositions of current and former CVR employees and directors, and that CVR may face "significant risk" of an SEC enforcement action.

SEC spokesman John Nester, Goldman spokesman Michael DuVally and Deutsche Bank spokeswoman Kerrie McHugh all declined to comment. CVR, Icahn and Wachtell did not immediately respond to requests for comment.